1. Field of the Invention
The present invention relates to a photosensitive composition and a pattern formation method for use in micropatterning of semiconductor devices, specifically, large-scale integrated circuits (LSIs).
2. Description of the Related Art
The manufacture of semiconductor devices such as LSIs makes use of a micropatterning technique performed by photolithography. This technique is done in accordance with the following processes. That is, a photo-resist film is first formed on a substrate, such as a silicon single-crystal wafer, by, e.g., spin coating and then the film is exposed. Thereafter, treatments such as developing and rinsing are performed for the resist film, thereby forming a resist pattern. Subsequently, the exposed wafer surface is etched by using the resist pattern as an anti-etching mask to form lines and windows with small widths, thereby forming a desired pattern.
In the manufacture of LSIs, as the packing density of LSIs has increased, a processing technique capable of forming finer patterns has been required in lithography. To meet this requirement, it has been conventionally attempted to shorten the wavelength of the exposure light source. As one such attempt, a lithography technique using as its light source deep UV, such as a KrF excimer laser (wavelength 248 nm) or an ArF excimer laser (wavelength 193 nm), has been examined.
Conventional resist materials, however, have an excessively large absorbance for deep UV. Therefore, it is impossible to make UV light sufficiently reach a portion apart from the surface of a resist film (e.g., an interface region between a resist film and a substrate) during exposure. Consequently, the chemical change which is due to the exposure does not occur satisfactorily throughout the total film thickness in the exposed portion of the resist film. The result is a nonuniform solubility in the direction of thickness with respect to a developing solution. The solubility especially in the portion apart from the surface of the resist film as described above is poor, and so the sectional shape of a resist pattern formed after development becomes triangular in that portion. This brings about a problem when the resultant resist pattern is to be used as an anti-etching mask; i.e., it is impossible to transfer a fine pattern of interest onto a substrate or the like.
As a resist material by which the above problem can be solved, a resist called a chemical amplification type resist has been proposed. The chemical amplification type resist is a photosensitive composition containing a compound which generates a strong acid upon being irradiated with light, i.e., a photo-acid generator, and a compound which changes into a hydrophilic substance if its hydrophobic group is decomposed by the acid generated. As a practical example of such resist material, U.S. Pat. No. 4,491,628 (1985) to H. Ito, C. G. Wilson, and J. M. J. Frechet discloses a positive resist containing a polymer which is obtained by blocking a hydroxyl group of poly(p-hydroxystyrene) by a butoxycarbonyl group, and an onium salt as the photo-acid generator. In addition, M. J. O""Brien, J. V. Crivello, SPIE, Vol. 920, Advances in Resist Technology and Processing, p. 42 (1988) discloses a positive resist containing an m-cresol novolak resin, a naphthalene-2carboxylic acid-tert-butylester, and a triphenylsulfonium salt as the photo-acid generator. Also, H. Ito, SPIE, Vol. 920, Advances in Resist Technology and Processing, p. 33 (1988) discloses a positive resist containing 2,2-bis(4-tert-butoxycarbonyloxyphenyl)propane or polyphthalaldehyde, and an onium salt as the photo-acid generator.
In each of these chemical amplification type resists, the acid generated by the photo-acid generator functions as a catalyst to bring about a chemical change efficiently inside the resist even with a small amount. When the resist film is exposed, therefore, the reaction proceeds sufficiently even in the interior of the film to which it is difficult to make a radiation reach compared with the film surface. This consequently makes it possible to form, after development is performed, a resist pattern having a rectangular sectional shape, particularly, a resist pattern in which the side surface of a line portion is steep and vertical.
In the above chemical amplification type resists, however, the amount of the acid generated in the exposed portion of the resist film is very small. Therefore, the resist is readily affected by the surrounding environment, especially the atmospheric oxygen and moisture and other atmospheric trace components on the surface of the resist film. This makes it difficult to stably form fine patterns. More specifically, a slight amount of dimethylaniline contained in the atmosphere deactivates an acid generated near the surface of the resist film upon irradiation of light. As a result, a so-called sparingly soluble layer whose rate of dissolution with respect to a developing solution is very low is formed on the surface of the resist film. It is reported that this sparingly soluble layer remains as an overhang on the surface of the resist pattern after exposure and development (S. A. MacDonald, N. J. Cleark, H. R. Werdt, C. G. Willson, C. D. Snyder, C. J. Knors, N. B. Deyoe, J. G. Maltabes, J. R. Morrow, A. E. MacGuire, and S. J. Hplmes, Proc. SPIE, Vol. 1466, 2 (1991)).
This sparingly soluble layer reduces the resolution of the resist, and an overhang formed on the resist pattern by the sparingly soluble layer adversely affects the etching accuracy of a semiconductor substrate region. To prevent the formation of this overhang-like sparingly soluble layer, as shown in FIG. 3A, exposure is performed after the atmospheric influence is reduced by forming a protective layer 8 on a resist film (Jpn. Pat. Appln. KOKAI Publication No. 4-2040848). Even by this method, however, an overhang cannot be completely removed, and an overhang-like sparingly soluble layer 9 as shown in FIG. 3B is formed on the side walls of a resist pattern 6. This method of forming a protective layer has problems in addition to the above problem; i.e., the method requires an additional coating device and degrades the workability because the number of processes is increased.
On the other hand, it is known that the resolution can be increased by adding to a chemical amplification type resist composition any of aniline-type, imidazole-type, pyridine-type, and ammonia-type derivatives each of which acts as a base with respect to the acid generated upon irradiation of light (Jpn. Pat. Appln. KOKAI Publication No. 5-127369). However, the miscibility of these amine-type compounds with respect to a low-molecular compound (to be described later) has not been reported yet.
In addition, in the formation of a pattern using the above chemical amplification type resist, when a resist film is formed, i.e., when a resist solution is coated on a substrate, a phase separation is sometimes caused in the film by the difference in molecular weight between components, resulting in a nonuniform concentration distribution of each component. As a result, the chemical change does not proceed uniformly in the exposed portion of the film, so a fine resist pattern with a rectangular sectional shape cannot be obtained stably.
The phase separation is described more specifically in H. Ito, J. Polymer. Sci.: Part A 24, 2971 (1986) which reports that in the synthesis of polyvinyl phenol, which is used as one component of a resist material, and in which phenolic hydroxyl groups are partially protected by tert-butoxycarbonyl, the phase separation occurs depending on the introduction rate of the protective groups of that polymer. The concentration distribution of a component in a resist film is described more specifically in M. Toriumi, M. Yanagimachi, and H. Masuhara, Proc. SPIE, Vol. 1466, 458 (1991).
It is also known that in the pattern formation using the chemical amplification type resist, the line width changes in correspondence with the baking temperature after exposure, and this decreases the sensitivity (J. Sturtevant, S. Holmes, P. Rablidoux, Advances in Resist Technology and Processing IX, SPIE, Vol. 1672, 114 (1992). Therefore, it is necessary to precisely control the baking temperature, resulting in a poor workability.
The present invention has been made in consideration of the above situations and has as its object to provide a photosensitive composition which has a high sensitivity and resolution with respect to particularly a light source with a short wavelength, which does not cause a phase separation in a film state, which is not readily influenced by the surrounding atmosphere, and by which fine resist patterns can be obtained stably.
The above object of the present invention can be achieved by photosensitive compositions according to the following first and second inventions.
The photosensitive composition according to the first invention contains (a) a polymer obtained by protecting an alkali-soluble group of an alkali-soluble polymer by a group which is unstable with respect to an acid, (b) a compound which generates an acid upon being irradiated with light, and (c) at least one compound which is selected from the group consisting of an imidazole compound, an alanine compound, an adenine compound, an adenosine compound, and a quaternary ammonium salt compound, and which increases miscibility in a resist film.
The photosensitive composition according to the second invention contains (a) a polymer obtained by protecting an alkali-soluble group of an alkali-soluble polymer by a group which is unstable with respect to an acid, (b) a compound which generates an acid upon being irradiated with light, and (d) a phenol compound.
In addition, a photosensitive composition according to the third invention contains (a) a polymer obtained by protecting an alkali-soluble group of an alkali-soluble polymer by a group which is unstable with respect to an acid, (b) a compound which generates an acid upon being irradiated with light, (c) at least one compound which is selected from the group consisting of an imidazole compound, an alanine compound, an adenine compound, an adenosine compound, and a quaternary ammonium salt compound, and which increases miscibility in a resist film, and (d) a phenol compound.
A manufacturing method of the present invention comprises the steps of forming, on a substrate, a resin layer containing the photosensitive composition according to any of the first, second, and third inventions as its main constituent, performing pattern exposure for the resin layer, baking the exposed resin layer, and developing the baked resin layer by using an alkali solution as a developing solution.
The photosensitive composition of the present invention corresponds to the positive chemical amplification type resist. That is, when this photosensitive composition is applied to a pattern formation process, an acid is generated from the component (b) in the exposed portion of the photosensitive composition. Upon baking, this acid acts as a catalyst to decompose the group which is contained in the component (a) and unstable with respect to the acid, thereby making the group produce an alkali-soluble group. As a result, the alkali solubility, i.e., the dissolution rate with respect to an alkali solution is raised in the component (a) in this exposed portion. Therefore, the exposed portion is selectively dissolved away by performing development by using an alkali solution. This makes it possible to form a fine pattern consisting of lines and spaces with predetermined widths.
The photosensitive composition according to the first invention contains, in addition to the components (a) and (b), the component (c), i.e., at least one compound selected from the group consisting of an imidazole compound, an alanine compound, an adenine compound, an adenosine compound, and a quaternary ammonium salt compound. In this photosensitive composition, this component (c) increases the resolution by acting as a base with respect to the acid generated by irradiation of light. The component (c) also increases the miscibility between the polymer as the component (a) and the low-molecular-weight compound as the component (b). This prevents a phase separation in a film state. Therefore, the concentration distribution of each component is maintained constant across the entire film, and the chemical change caused by exposure or the like proceeds evenly. In the present invention, fine patterns are formed by this effect of high miscibility. In addition, since the component (c) increases the alkali solubility in the exposed portion of the film, the formation of an overhang-like sparingly soluble layer can be prevented without forming any protective film on the resist film. Furthermore, even if the time period from the exposure to the heat treatment performed by baking is prolonged in the pattern formation process, the photosensitive composition according to the first invention controls diffusion of the acid generated by the exposure, making the formation of fine patterns possible.
The photosensitive composition according to the first invention, therefore, keeps its high sensitivity and resolution inherent in the chemical amplification type resist, is stable throughout the whole pattern formation process, and provides a fine pattern having a rectangular sectional shape. As described above, the first invention makes it unnecessary the use of a protective film which is required in conventional chemical amplification type resists. In addition, it is also possible to omit a protective film formation step, and this reduces the total number of steps in the manufacturing process.
The second photosensitive composition of the present invention is a composition containing, as its essential components, (a) a polymer obtained by protecting an alkali-soluble group of an alkali-soluble polymer by a group which is unstable with respect to an acid, (b) a compound which generates an acid upon being irradiated with light, and (d) a phenol compound.
This second photosensitive composition also corresponds to the positive chemical amplification type resist. Therefore, when applied to the pattern formation process, the second photosensitive composition provides fine patterns in accordance with the same mechanism as that of the above first composition.
The second photosensitive composition contains the component (d), i.e., a phenol compound, in addition to the components (a) and (b). In the second photosensitive composition, this phenol compound increases the miscibility between the polymer as the component (a) and the low-molecular-weight compound as the component (b). For this reason, as in the above first composition, no phase separation takes place in a film state, the concentration distribution of each component is kept constant across the entire film, and the chemical change caused by exposure or the like proceeds evenly. Especially in this second photosensitive composition, it is assumed that a hydroxyl group contained in the phenol compound reacts with the alkali-soluble group which is not protected but remains in the polymer as the component (a), thereby preventing separation of the component (a) with a high molecular weight. In addition, since the phenol compound increases the alkali solubility in the exposed portion of the film, the alkali solubility of the resist film can be increased compared to those of conventional resist films. Therefore, the formation of a sparingly soluble layer on the film surface can be prevented. Furthermore, since the photosensitive composition according to the second invention has a large focus margin, fine patterns can be formed even if a focusing error occurs during the exposure. This makes it possible to increase the working efficiency. The second photosensitive composition also can make it unnecessary the use of a protecting film, which is required in conventional chemical amplification type resists, by controlling the amounts of the photo-acid generator and at least one compound selected from the group consisting of an imidazole compound, an alanine compound, an adenine compound, an adenosine compound, and a quaternary ammonium salt compound.
As described above, the second photosensitive composition of the present invention keeps its high sensitivity and resolution inherent in the chemical amplification type resist, is stable throughout the whole pattern formation process, and provides a fine pattern having a rectangular sectional shape.
The photosensitive composition according to the third invention contains a phenol compound as a component (d), in addition to the components (a), (b), and (c) of the first composition. Since the components (c) and (d) further increase the miscibility between the polymer as the component (a) and the low-molecular-weight compound as the component (b), a phase separation in a film state can be prevented. In addition, the combination of the components (c) and (d) greatly raises the dissolution rate with respect to an alkali solution in the exposed portion of the film. This prevents the formation of a sparingly soluble layer on the film surface and also improves the contrast between the exposed and unexposed portions.
The photosensitive composition according to the third invention, therefore, keeps its high sensitivity and resolution inherent in the chemical amplification type resist, and is more stable than the composition according to the first invention throughout the whole pattern formation process. Consequently, the third photosensitive composition provides a fine pattern having a rectangular sectional shape, in which not only the formation of an overhang-like sparingly soluble layer on the film surface is prevented, but also a standing-wave effect on the side surfaces of the film is reduced or eliminated (the standing-wave effect is an effect by which a wave having a predetermined period is produced on the side surfaces of the resist because energy stored in the resist film varies in the direction of depth of the resist film due to reflection of exposure light from the resist surface and an interference effect of light components reflected by the resist film and the surface of a substrate).
The photosensitive composition according to the third invention also can make the use of a protective film unnecessary. Consequently, the total number of steps in the manufacturing process can be reduced since a protective film formation step can be omitted.
The present invention further provides a pattern formation method using any of the first, second, and third photosensitive compositions described above. That is, the pattern formation method of the present invention comprises the steps of forming, on a substrate, a resin layer containing any of the above first, second, and third photosensitive compositions as its main constituent, exposing a pattern on the resin layer, baking the exposed resin layer, and developing the baked resin layer by using an alkali solution as a developing solution.
The pattern formation method of the present invention can increase the working efficiency because the sensitivity does not change even if the baking temperature changes.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.